Metal halide lamps are known in the art and are described, for example, in EP0215524, WO2006/046175, and WO05088675. Such lamps operate at a high pressure and comprise ionizable gas fillings of, for example, NaI (sodium iodide), TlI (thallium iodide), CaI2 (calcium iodide), and/or REIn. REIn refers to rare-earth iodides. Characteristic rare-earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3, and LuI3. An important class of metal halide lamps includes ceramic discharge metal halide lamps (CDM-lamps), which are described in the above-mentioned documents.
EP0215524 discloses a high-pressure mercury vapor discharge lamp having a discharge vessel of gastight radiation-transmitting ceramic material, provided with a filling comprising a rare gas, mercury, sodium halide and thallium halide. The wall load has a value of at least 25 W/cm2. The ratio between the effective internal diameter of the discharge vessel and the spacing between two electrodes is in a specific range.
WO2006/046175 discloses a metal halide lamp comprising a discharge vessel enclosing a discharge space containing an ionizable gas filling comprising Hg in a quantity of mass and at least a metal halide, which discharge space accommodates two electrodes whose tips have a mutual interspacing so as to define a discharge path between them, and which discharge space has a length, measured along the discharge path, and a largest diameter square thereto, wherein the ratio of the discharge space and the largest diameter is in a specific range.
WO05088675 discloses a metal halide lamp comprising a discharge vessel surrounded with clearance by an outer envelope and having a ceramic wall which encloses a discharge space filled with a filling comprising an inert gas, such as xenon (Xe), and an ionizable salt, which discharge space accommodates two electrodes whose tips have a mutual interspacing so as to define a discharge path between them, with the special feature that said ionizable salt comprises NaI, TlI, CaI2 and X-iodide, wherein X is selected from the group comprising rare-earth metals. In a specific embodiment of WO05088675, X is one or more elements selected from the group comprising Ce, Pr, Nd.
U.S. Pat. No. 7,180,229 discloses a high-pressure lamp with a base and an inner vessel sealed in a vacuum-tight manner and surrounded by a sleeve part, the base having electric terminals which support the inner vessel on one side and the sleeve part on the other side, the reflector having a rotationally symmetrical design and a contour divided into at least two zonal layers, whose axial height is dimensioned in such a way that each zone captures at least 35% of the light intensity emerging from the center of the inner vessel, with a first zone reflecting back at least 90% of the light incident on it at positive angles in relation to the lamp axis, and a second zone reflecting back at least 90% of the light incident on it at negative angles in relation to the lamp axis, while the inner vessel contains a metal halide filling, and the lamp has a specified average color temperature. The lamp is a metal halide lamp for general lighting purposes, whose filling may contain halides of, inter alia, Na, Sn, Ca, Tm, Tl.
US2003141818 discloses a metal halide lamp with red emission equivalent to or exceeding that of a black body source of equal correlated color temperature. A metal halide chemistry containing CaI2 plus a complexing metal halide of AlI3 or GaI3 is used to substantially increase the red emission of a metal halide lamp. The inclusion of TlI in the fill chemistry is also important in influencing Ca to preferably emit atomic and molecular red radiation of the visible spectrum while suppressing blue radiation. Optionally, a shroud of neodymium-doped glass is also used to significantly filter transmission of yellow light, thereby further improving the proportion of red emission while maintaining a sufficiently white color and satisfactory general color rendering.